Preheat the oven to 350 degrees F. Grease a 9-inch baking pan with butter or oil and keep this aside.

Add the dry ingredients in one bowl (flour, cocoa powder, baking powder, soda, salt, coffee powder, sugar) and whisk.

Add egg, milk, oil, and vanilla extract and whisk to combine.

Dissolve coffee powder in hot water and add this to the batter. Mix until just combined.

Pour the cake batter into a greased pan and bake for 25 to 30 minutes or until the toothpick inserted comes out clean.

Let the cake cool in a pan for 5 to 10 minutes. Remove the cake from the pan and let it cool down completely before layering it with buttercream. (Keep the cake in the refrigerator for 30 minutes if needed)

Add chocolate chips to a microwave-safe bowl and microwave for 30 seconds. Stir and continue heating for another 15 seconds or until the chocolate melts and is smooth. Let the chocolate cool; stir it occasionally.

In a separate bowl, add butter and, using a held mixer or stand mixer, beat until creamy.

Add salt, confectioner's sugar in increments, vanilla extract, and whisk until combined.

Now add cooled melted chocolate and whisk until thoroughly combined.

Drizzle in heavy cream if needed and continue whisking (for 2 to 3 minutes) until it is stiff and creamy. (Add heavy cream to adjust consistency. Do not add everything at once. Because if frosting becomes thin, it will be difficult to correct it).

Keep it refrigerated until further use. This chocolate buttercream stays well for up to a week when refrigerated in an airtight container.

Melt chocolate chips and cream together on a double boiler, stirring until smooth and silky. Keep this aside.

Level the top of the cake using a knife to create a flat surface if required.

Using a 3-inch cookie cutter, form 6 mini cakes. Slice each mini cake into thirds using a sharp knife.

Add a large scoop of chocolate buttercream to the bottom third of the cake to assemble. Spread lightly with a spatula. Place the middle third of the cake and gently press until it levels.

Add another large scoop of chocolate buttercream over the middle third. Spread lightly using a spatula as before, and place the top third of the cake and gently press it.

Using a straight edge of a spatula, remove excess cream that has squeezed out between the layers and simultaneously fill any gaps (crumb-coat). Transfer this to a wire rack.

Repeat the process for the remaining five mini cakes. Now, you can optionally transfer the cakes to the freezer for about 10 minutes. This helps solidify the buttercream slightly and provides the cakes with some strength.

Next, pour the chocolate ganache over the mini cakes and give the desired finish. I have gone for a rustic semi-naked finish.

Garnish it with some shavings of dark chocolate. You can make these mini-cakes ahead of time and refrigerate them until you are ready to serve.